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Former White House chief strategist Steve Bannon (C) arrives to speak at a closed-door meeting with the House Intelligence Committee Feb. 15, 2018 on Capitol Hill in Washington, DC.

Steve Bannon, who served as President Donald Trump’s chief strategist, was interviewed by special counsel Robert Mueller over multiple days this week, NBC News has learned from two sources familiar with the proceedings.

Bannon spent a total of some 20 hours in conversations with the team led by Mueller, who is investigating possible collusion between the Trump campaign and Russia as well as other issues that have arisen around the probe.

Bannon left his job as a senior White House adviser in August and returned to a leadership role at Breitbart, the right-wing news site based out of Washington. But he fell out of favor with the site’s financial backers, the Mercer family, after criticizing the president and his family in "Fire and Fury,” a book about the Trump administration published earlier this year by author Michael Wolff.

After a more than four-week stalemate, Bannon also returned to Capitol Hill Thursday to resume his interview with the House Intelligence Committee, which was halted when he earlier refused to answer key questions in the Russia probe.

FBI Deputy Director Fired Two Days Before Retirement

Attorney General Jeff Sessions fired former Deputy Director Andrew McCabe, who was a regular target of President Donald Trump's anger and criticism.